1. Field of the Invention
This present invention relates to a semiconductor memory device which is reprogrammable, and particularly relates to a reprogrammable semiconductor memory device having a page latch.
2. Related Art
In some of a nonvolatile semiconductor memory device (EEPROM) which is reprogrammable by one byte to by a few tens bytes (for one page), one latch circuit (page latch circuit) for retaining one page data is provided for every bit line. In this specification, the nonvolatile semiconductor memory device is called a semiconductor memory device having a page latch.
An operation of a conventional nonvolatile semiconductor memory device having a page latch will be explained. FIGS. 18a to 18c indicate data flow diagrams at a data loading operation, at a programming operation and at a read operation in a conventional semiconductor memory device having a page latch, respectively. First of all, as shown in FIG. 18a, one-page program data are loaded to a page latch. When one-page program data are stored in the page latch, typically one page data stored in memory cells are erased.
As shown in FIG. 18b, the one-page program data in the page latch are simultaneously written to the one-page memory cells, whose previous data have been erased. Also, when a data in the memory cell is read out, as shown in FIG. 18c, a selected memory cell is connected to a read out circuit and the data is read from the selected memory cell.
However, once a data loading operation is started, the operation continues to a data erasing operation and a data programming operation automatically in the conventional nonvolatile semiconductor memory device with a page lath. Also, in the data reading out operation, the conventional nonvolatile semiconductor memory device only has a mode in which the operation reads out data programmed to the memory cell.
In such conventional nonvolatile memory devices with page latches, when a data is programmed to a memory cell and the programmed data is read out from the memory cell, and assuming that the data which is read out includes an error, it is very hard to determine whether the data which is programmed to the memory cell has included the error or the data which is read out from the memory cell was broken at the read out circuit.
Also, when you test the page latch and the read out circuit in the conventional nonvolatile semiconductor memory device, you need a very long time to test because a data is programmed to a memory cell automatically.